1. Field of the Invention
The present invention relates generally to the art of lead-acid batteries, and more particularly to processes for producing batteries having extended shelf-life and to the batteries produced by such processes. The invention also relates in its most preferred embodiment to the use of a gelled desiccant contained within a battery housing to maintain the specific gravity of the electrolyte of a dampened, formed, battery within a predetermined specific gravity range. In the most preferred embodiment, the desiccant and absorbed water is later used to form the reconstitution electrolyte for the battery.
2. Description of the Prior Art
Attempts have been made for many years to improve the storage life of lead-acid batteries. Using the easiest example for explanation, it would be highly desirable to be able to manufacture batteries for distribution through the automobile aftermarket, which batteries would be stable over long periods of time, e.g., 2-4 years. Such batteries would make inventory control easier for those involved in the distribution system, and long shelf-life would be especially beneficial if little or no additional charge would be required at the time the battery is placed into service.
While such efforts have taken place, two principal types of systems have been developed, one by the assignee of the present invention, known as the damp storage system. In this battery production technique, the battery is prepared in the normal fashion and formed to the desired voltage with wet plates and separators. The acid is then dumped and removed in a spin process, so that only a residual quantity of electrolyte is maintained. While the process has achieved some success, the storage life is not as long as desired Damp batteries are activated by the addition of acid, and typically a charge is provided by the installer of the battery to bring the battery back to its desired voltage. See, U.S. Pat. No. 3,652,341 issued Mar.28, 1972 to Halsall, et al.; U.S. Pat. No. 3,948,680 issued Apr. 6, 1976 to Mao, et al.; and U.S. Pat. No. 3,988,165 issued Oct. 26, 1976 to Mao, et al.
A dry charge technique has also been developed, in which charged plates and separators are prepared outside of the battery housing and are placed into the housing in a dry condition. These batteries have a somewhat longer shelf-life than the damp variety and are also acid activated. See, G. Vinal, Storage Batteries, 4th Ed., John Wiley & Sons, Inc., New York, 1955, p. 41, and H. Bode, Lead-Acid Batteries, John Wiley & Sons, Inc., New York, 1977, pp. 277-279. While dry-charged batteries probably offer the longer shelf life, their production requires extra processing involving large quantities of water. This water must be treated before it can be released into the municipal sewer system. The cost of this treatment and the resultant environmental concerns tend to make the dry-charge system undesirable.
The reasons why long shelf-life lead-acid batteries have been difficult to prepare relate to the complex electrochemistry of such systems and the battery self-discharge reactions which were first reported in the late 19th Century. Since that time, the reactions which are known to limit the shelf-life in lead-acid batteries have been well defined and are listed below:
At the positive electrode, they are
Oxygen evolution: EQU PbO.sub.2 +H.sub.2 SO.sub.4 .fwdarw.PbSO.sub.4 +H.sub.2 O+1/2 O.sub.2[ 1]
Hydrogen recombination: EQU PbO.sub.2 +H.sub.2 +H.sub.2 SO.sub.4 +.fwdarw.PbSO.sub.4 +2H.sub.2[ 2]
Oxidation of organic contaminants: EQU PbO.sub.2 +organics+H.sub.2 SO.sub.4 .fwdarw.PbSO.sub.4 +H.sub.2 O+CO.sub.2 +oxidized organic products [3]
Grid corrosion: EQU PbO.sub.2 +Pb+2H.sub.2 SO.sub.4 .fwdarw.2PbSO.sub.4 +2H.sub.2 O [4]
[A variety of other corrosion reactions can occur as well.]
Oxidation of grid alloying metals, such as antimony: EQU 5PbO.sub.2 +2Sb+6H.sub.2 SC.sub.4 .fwdarw.(SbO.sub.2).sub.2 SO.sub.4 +5PbSO.sub.4 +6H.sub.2 O [5]
(Similar reactions can be written for other metals.)
Sulfation of "apparent PbO": EQU PbO+H.sub.2 SO.sub.4 .revreaction.PbSO.sub.4 +H.sub.2 O [6]
At the negative electrode, two more reactions can occur
Hydrogen evolution: EQU Pb+H.sub.2 SO.sub.4 .fwdarw.PbSO.sub.4 +H.sub.2 [ 7]
Oxygen recombination: EQU Pb+H.sub.2 SO.sub.4 +1/2O.sub.2 .fwdarw.PbSO.sub.4 +H.sub.2 O [8]
These various reactions, and their effect on acid-starved lead-acid batteries, are discussed in detail in the 1982 article of Bullock, et al. printed in Volume 129, No. 7, Jul., 1982 issue of The Journal of Electrochemical Society, pages 1393-1398. A copy of this article is provided with the specification.
As pointed out in the Bullock, et al. article, which of the many reactions will dominate at any particular time depends upon a number of factors, including the design of the battery and the materials used. Moreover, a relationship is noted between the concentration of the acid and time and temperature. It is noted that acidstarved batteries, such as those prepared using the damp storage system, demonstrate a relatively predictable decline in open-circuit voltage over time, and a change in the slope of the curve indicates changes in reactions which occur depending upon the rate of grid corrosion and gas evolution It is a further understanding of these reactions and the discovery by the present inventors of a technique for controlling the reactions which form the basis for the present invention.
Another item of prior art which should be discussed in the context of the present invention is a patent owned by the assignee of the present invention, i.e., U.S. Pat. No. 3,556,860 issued Jan. 19, 1971 to Amlie for "Storage Battery and Electrolyte Precursor Therefor". In this patent, the patentee suggests the use of a gelled sulfuric acid composition for forming a battery electrolyte to be used in combination with dry charge batteries for the on-the-site preparation of ready-to-use lead-acid batteries. The preferred gelled sulfuric acids described in this application are those which include oxides of boron and phosphorous together with sulfuric acid. While one of the patent drawings is used to define the range of acids covered by the claims of the Amlie patent, one example thereof is a gel having a molar ratio of 1.4 H.sub.3 PO.sub.4 /4.0 H.sub.3 BO.sub.3 /100.0 H.sub.2 SO.sub.4. That particular material, prepared by mixing boric acid, phosphoric acid and sulfuric acid, resulted in a firm gel after about 10 hours at 80.degree. C. (see Example 1 of the patent).
The gel prepared by Amlie was contained in a bag, which the patent indicates has no particular structure since the gelled sulfuric acid has sufficient rigidity to retain its slab-like shape. Perforations were provided to permit the ready escape of the electrolyte when the gel was later mixed with water. The Amlie gel did not perform any type of desiccant function, since it was employed with dry-charged systems as described earlier.
Other patents have disclosed the use of gelled acids for dry charge batteries which are mixed with water at the time of use. See, for example, Solomon U.S. Pat. No. 3,067,275, issued Dec. 4, 1962 for "Storage Battery Electrolyte"; Little U.S. Pat. No. 3,530,002 issued Sep. 22, 1970 for "Water-Activated, Dry Charge Lead-Acid Storage Battery Utilizing Gelled Sulfuric Acid Electrolyte Precursor and Method of Activating Same"; and Lauck U.S. Pat. No. 3,586,539 issued Jun. 22, 1971 for "Lead Accumulator With Dry Storage Stable Charged Electrode Plates". Each of the foregoing patents indicates that the gelled material is used to maintain the dry charge state and does not teach or suggest the use of a gelled desiccant for control of the specific gravity of an electrolyte in a damp system or prolonging shelf-life by water removal therefrom. The increase of battery shelf-life using a material which is subsequently incorporated into the battery system would represent a significant advance in this technology.